#include <cstdio>

int main(int argc, char const* argv[])
{
    double x, y;
    scanf("%lf", &x);
    if (x >= 0 && x < 5)
        y = x * -1 + 2.5;
    else if (x >= 5 && x < 10)
        y = 2 - 1.5 * (x - 3) * (x - 3);
    else if (x >= 10 && x < 20)
        y = x / 2 - 1.5;
    printf("%.3lf", y);
    return 0;
}